Study of floods on the rivers of the Kaliningrad region at the beginning of 2020 in the absence of snow cover and ice

. To develop measures for engineering protection of territories from flooding, it is necessary to identify spatiotemporal patterns in the formation of floods, since they are the cause of devastating floods in recent years. The paper presents the results of an analysis of flood phenomena on the rivers of the Kaliningrad region at the beginning of 2020. An array of observational data on daily water flows in rivers was processed, and the main components of the water balance of river basins were assessed by month (amount of precipitation, evaporation layer, monthly river flow layer, change in basin moisture reserves). It was found that the greatest amount of runoff was in February, while the highest amount of precipitation was in June; the highest runoff coefficient was in March and February (0.41), and in June it was only 0.03. Presumably, the main reasons for such phenomena are the distribution of the evaporation layer within the year and a decrease in the permeability of the upper soil layer for precipitation at low temperatures.


Introduction
In connection with climate change, much attention is paid to the study of winter floods on European rivers ( [1][2][3][4][5][6][7] and the literature therein).Thus, in [3] it is noted that the study of flood runoff is an important task, since in recent decades the most destructive floods in the European territory of Russia have been associated with floods.In [1], regional patterns of changes in observed river flood discharges in Europe over the past 50 years are considered.The authors of [1] suggest that these changes are caused by climate change and need to be taken into account when managing flood risks.The authors of [2] analyzed the timing of river floods in Europe over the past five decades based on data from 4262 hydrometric observation stations.Research [2,4] made it possible to identify patterns of changes in the time of passage of floods on rivers.In [5][6][7], the causes and consequences of floods in Europe over recent decades are studied in detail, and the meteorological and hydrological characteristics of these events are considered.
The purpose of this work is to study flood phenomena on the rivers of the Kaliningrad region at the beginning of 2020.

Materials and methods
The initial data for the study were the results of observations of daily water flows in the rivers of the Kaliningrad region.The data set was taken from the automated information system of state monitoring of water bodies.To analyze the hydrological characteristics, the components of the water balance of the river basin were assessed by month: R i is the amount of atmospheric precipitation for the i-th month, E i is the evaporation layer for the month, h i is the layer of monthly river flow, ∆V i is the change in basin moisture reserves (positive or negative).The water balance equation of a river basin can be written in the following form [8][9]: Total evaporation for the year, as in [1,3], was estimated using the formula: Where E 0 is evaporation (maximum possible evaporation), calculated from the average annual air temperature T: ( It is shown in [10] that for river catchments in North-West Russia, quite satisfactory calculation results using formula (3) are obtained with the following coefficient values: a0 = 329; a1 = 62; a2•= 2.14.They were used in this article.
As is known, to a first approximation, the intra-annual change in the evaporation layer can be estimated from the values of the average monthly humidity deficit d i : The average monthly humidity deficit is calculated using the formula: Where U i is the average air humidity in the i-th month, %; e i -average partial pressure of water vapor in the i-th month, Pa; p(T i ) -dependence of pressure (Pa) of saturated water vapor on air temperature (°C).
The runoff coefficient was determined by the formula: To perform calculations using the above formulas, weighted average (over the drainage basin) values Ψ = (T, U, R) are required.
Specialized data sets for climate research were used as a source of temperature and humidity values, and amounts of precipitation.Average monthly data for two weather stations are presented in Table 1.

Results and discussion
In Figures 1and 2 show the hydrographs of the Instruch and Zlaya rivers.At first glance, they have the usual appearance for the region, a spring flood with several peaks.The actual picture of the phenomenon is completely different.The fact is that in 2020 in the river basins of the Kaliningrad region, including the river.Instruch, there was no snow cover, and there was no freeze-up.In Figure 3 and 4 show the change in the modulus of river flow in the Kaliningrad region in the first half of 2020.It can be seen that the river hydrographs are very similar.This is also evidenced by the high pair correlation coefficients at the bottom of the Table 2.But there are also differences.Thus, unlike the Zlaya and Instruch rivers, on the E3S Web of Conferences 463, 02009 (2023) EESTE2023 https://doi.org/10.1051/e3sconf/202346302009Mamonovka River there is no peak water flow in the second ten days of February.On the Lava River, unlike the Angrapa and Pregolya rivers, there is a peak water flow in the third ten days of February.Most likely, such differences are due to the fact that a significant part of the drainage basin of both the Mamonovka River and the Lava River is located on the territory of the Republic of Poland, where there are differences in climatic characteristics from the Kaliningrad region.In Figure 5 shows the course of average daily air temperatures at the Sovetsk weather station in the first half of 2020.Temperatures at other weather stations in the Kaliningrad region change in a similar way.In Figure 6 shows that the amount of precipitation in the second ten days of May was approximately the same as in the third ten days of January, and in the second ten days of February it was slightly less.However, the modulus of river flow in the Kaliningrad region was much greater in February than in May.An analysis of the reasons for this phenomenon can be carried out using the data in Figure 5.In the first ten days of February, negative temperatures were observed (especially at night), which led to low evaporation and low

Conclusion
To analyze the resulting phenomenon, an assessment was made of the components of the water balance of the river basin by month.In Figure 9 presents the results of hydrometeorological indicators by month of 2020 in the Instruch River basin.
According to Figure 9a shows that the largest runoff layer was in February (34.0 mm), then in March and January; in other months -less than 4.5 mm.Whereas according to Fig. 9b, the highest amount of precipitation was in June (106.6mm), when the monthly runoff layer was only 3.2 mm.The highest runoff coefficient (Figure 9f) was in March (0.56) and February (0.41), in June it was only 0.03.We can name two, in our opinion, main reasons for this phenomenon.The main reason is the distribution of the evaporation layer within the year (Figure 9d), it is determined by temperature (Figure 9c) and air humidity.In June this layer was E 6 = 102 mm, while in February it was much less E 2 = 26.5 mm.Another reason for high runoff coefficients in the first months of the year is the decrease in the permeability of the topsoil to precipitation at low temperatures.

Fig. 5 .
Fig. 5. Average daily air temperatures at the Sovetsk weather station in the first half of 2020.

E3S
Web of Conferences 463, 02009 (2023) EESTE2023 https://doi.org/10.1051/e3sconf/202346302009permeability of the soil layer.As a result, there were high runoff coefficients of the rivers of the Kaliningrad region in February and in the second ten days of January and very low in May (Figures 7and 8).

Fig. 6 .
Fig. 6.Precipitation amounts by decade in the first half of 2020 in the Zlaya River basin (weighted average at the Sovetsk weather station and Chernyakhovsk weather station).

Fig. 7 .
Fig. 7. Flow coefficient by decade in the first half of 2020 for the Zlaya River.

Fig. 8 .
Fig. 8. Flow coefficient by decade in the first half of 2020 for the Instruch River.

Fig. 9 .
Fig. 9. Hydrometeorological indicators for the months of 2020 in the Instruch river basin: a -river flow layer; b, c -weighted average amounts of precipitation and temperature, respectively; d, eassessment of the evaporation layer and changes in water reserves, respectively; f -runoff coefficient.